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  Power amplifierUSPTO Application #: 20080007342Title: Power amplifier Abstract: A power amplifier is described that includes an amplification circuit and a linear regulator. The amplification circuit amplifies an input signal to produce an output signal and provides a feedback signal for tracking the expected power requirements of the output signal. The linear regulator is connectable for receiving as inputs the feedback signal from the amplification circuit and a plurality of fixed voltage supply rails. The linear regulator provides as an output at least one voltage supply to the amplification circuit. In at least one mode of operation, the at least one voltage supply provided to the amplification circuit tracks the expected output requirements of the amplification circuit between two of the plurality of fixed voltage supply rails. (end of abstract) Agent: Bereskin And Parr - Toronto, ON, US Inventors: Atron Lo, Jeliazko Stoyanov Batchvarov USPTO Applicaton #: 20080007342 - Class: 330297 (USPTO) The Patent Description & Claims data below is from USPTO Patent Application 20080007342. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001]The present invention relates to power amplifiers, and more specifically to low power linear power amplifiers. BACKGROUND OF THE INVENTION [0002]Audio signals typically have a large dynamic range. For example, audio signals often have a peak-to-average magnitude ratio of 8-to-1 or even higher, which means that the average value of an audio signal is much smaller than possible peak values. However, the peaks in an audio signal are relatively infrequent and at most times an audio signal has a magnitude close to a relatively low average magnitude. [0003]A power amplifier for amplifying an audio signal must be capable of producing an output signal with corresponding high peaks while most often producing an output audio signal with a comparatively low magnitude. Typically for a power amplifier, the DC bias current required to maintain the necessary operating range within the active amplifying devices to produce peak output power is significantly higher than the DC bias current required to amplify the audio signal at or close to the relatively low average level. Thus, given that peaks in an audio signal are infrequent, the bias current will be higher than required most of the time which in turn leads to low efficiency if peak power is always made available, such as in a typical class AB amplifier. In contrast it would be preferable if the available output power could be adjusted to track the requirements of the input signal. Those skilled in the art will appreciate that headroom is defined as the difference between the supplied power and that of the output audio signal. SUMMARY OF THE INVENTION [0004]In a first aspect, at least embodiment described herein provides a power amplifier comprising an amplification circuit for amplifying an input signal to produce an output signal and providing a feedback signal for tracking the expected power requirements of the output signal; and a linear regulator, connectable for receiving as inputs the feedback signal from the amplification circuit and a plurality of fixed voltage supply rails and providing as an output at least one voltage supply to the amplification circuit, wherein in at least one mode of operation the at least one voltage supply provided to the amplification circuit tracks the expected output requirements of the amplification circuit between two of the plurality of fixed voltage supply rails. [0005]The power amplifier may further comprise a multi-level power supply for providing a plurality of fixed voltage supply rails to the linear regulator. [0006]The linear regulator may comprise positive and negative voltage rails; a voltage divider across the positive and negative voltage rails including two resistors R1 and R2 connected at a ground potential; a linear control circuit for tracking the expected output requirements of the amplification circuit between two of the plurality of fixed voltage supply rails; and a coupler connected between the linear control circuit and the voltage divider for selectively coupling the linear control circuit to the voltage divider. [0007]The coupler may include the combination of a transistor and a diode connected between the linear control circuit and the voltage divider, with the anode of the diode connected to the voltage divider. [0008]In a first mode of operation the coupler may be configured to operate as an open circuit, and in a second mode of operation the coupler can be configured to operate as a tunable resistor. [0009]The positive voltage rail may be connectable directly to a terminal of a multi-level power supply that provides at least some of the plurality of fixed voltage supply rails. [0010]The linear control circuit may be connectable to receive as inputs the feedback signal from the amplification circuit and the level of the at least on voltage supply provided by the linear regulator to the amplification circuit. [0011]The linear control circuit may comprises a transistor connectable to the voltage divider of the linear regulator; an offset stage for prebiasing the transistor to an on state without the transistor being coupled to the voltage divider; and a comparator for summing respective currents derived from the feedback signal and the level of the at least one voltage supply rail of the amplification circuit and comparing the respective voltages of each and adjusting the bias on the transistor in response to fluctuations in the feedback signal and the level of the at least one voltage supply rail. [0012]The comparator may be configured to bias the transistor to provide additional supply voltage to the amplification circuit when the expected power requirements for the output signal is above the magnitude of a first voltage supply rail and below the magnitude of a second voltage supply rail. [0013]The comparator may be configured to bias the transistor to not provide additional supply voltage to the amplification circuit when the expected power requirements for the output signal is below the magnitude of the first voltage supply rail. [0014]The linear control circuit may further comprise a first current converter connected to the comparator, the first current converter being configured to receive the feedback signal and providing a corresponding first current signal to the comparator; a second current converter connected to the comparator, the second current converter being configured to receive one of the at least one voltage supply rail from the amplification circuit and providing a corresponding second current signal to the comparator, and a drive stage connected to the comparator and the transistor, the drive stage being configured to receive the output of the comparator to bias the transistor. [0015]The linear control circuit may further comprise a voltage clamp connected between the output of the drive stage and the input of the transistor, the voltage clamp being configured to limit the amount of bias of the transistor. [0016]The power supply may comprise a transformer including primary and secondary windings; a control circuit coupled to respective points on both the primary and secondary windings, the control circuit being operable to maintain a fixed voltage level on the primary winding irrespective of voltage level changes on the secondary winding; and a snubber circuit connected to the primary winding of the transformer, the snubber circuit including a combination of first and second diodes, a capacitor and a resistor, wherein the first diode and the capacitor are connected in series across the primary winding, with the capacitor connected to one end of the primary winding, and one end of the resister is connected at a node connecting the diode and the capacitor in series, and the second diode is connected between the other end of the resistor and ground, wherein the discharge path of the capacitor is through the resistor to ground. [0017]The power supply may further comprise a transistor connected between the control circuit and the one end of the primary winding connected to the capacitor. [0018]The power supply may further be arranged in a flyback configuration. [0019]The power supply may further comprise a second capacitor and a third diode connected in series between a first terminal of the secondary winding and a center tap of the secondary winding with the cathode of the third diode being connected to the center tap; a third capacitor and a fourth diode connected in series between the first terminal of the secondary winding and a second terminal of the secondary winding with the cathode of the fourth diode being connected to the second terminal; and a fifth diode having a cathode connected to the anode of the third diode, wherein the power supply provides a first voltage supply rail connectable to the first terminal of the secondary winding, and a second voltage supply rail connectable at the anode of the fifth diode. [0020]In another aspect, at least one embodiment described herein provides a method of adjusting the supply voltages provided to an amplification circuit of a power amplifier based on a plurality of fixed supply voltage rails, wherein the method comprises: [0021]generating a feedback signal indicative of the expected power requirements of an output signal produced by the power amplifier; Continue reading... Full patent description for Power amplifier Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Power amplifier patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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